Fuel cells find many applications as substitutes for standard batteries in many electrical devices. The cores of fuel cells of PEMFC type (Proton Exchange Membrane Fuel Cell) and DMFC type (Direct Methanol Fuel Cell), also known as MEAs (Membrane/Electrode Assemblies), are formed from a polymer electrolyte membrane and two electrodes (anode and cathode) placed on each face of this membrane.
MEAs enable conversion of the chemical energy of a reaction into electrical energy, for example that of formation of water from hydrogen (H2) for PEMFC or of methanol for DMFC and oxygen gas (O2).
The membrane of PEMFC and DMFC fuel cells must satisfy 3 functions:                conduction of the ionic species and more specifically of protons,        separation of the anode and cathode reagents,        electronic insulation.        
In order to ensure all these functions, the membrane must be a good proton conductor, with a conductivity of between 5×10−2 and 10−1 S/cm under the operating conditions. It must be sparingly permeable to reagents (H2 and O2 in the case of PEMFC or methanol and O2 in the case of DMFC), and must preferably have a permeability of less than 10−14 mol/(m·s·Pa) for each of the reagents under the operating conditions. Finally, it should be a good electronic insulator.
Moreover, the membrane must be stable (chemically, physically) under the operating conditions of the fuel cell. The criteria that influence the stability of the materials are temperature, the amount of water, the activity of the reagents (especially H2, or O2 in the case of PEMFC), the electrical potential (V) and the pH. The most extreme conditions that may be observed in a fuel cell of PEMFC type are:                pH from 0 to 1,        temperature: 120° C.,        100% relative humidity,        anode: PH2=4 bar, Va=0 V,        cathode: PO2=4 bar, Vc=1.2 V.        
Finally, this material must be inexpensive, nontoxic and easy to handle.
At the present time, the most advanced proton exchange membrane fuel cells (PEMFCs), whether they are commercially available or incorporated into demonstrators, are made with perfluorosulfone polymer electrolytes of Nafion® or Hyflon® type. Specifically, this type of polymer simultaneously has the best performance qualities and the longest lifetime. However, the performance qualities achieved with these membranes are still insufficient, irrespective of their uses (portable, stationary, transport).
Application in the field of transport is the most constraining from the point of view of the specifications (cost, working temperature, performance, lifetime). Ideally, for an application in motor vehicles, it would be necessary to have available a PEMFC that operates between −20° C. and 120° C. in sparingly humidified gases (between 0 and 50% relative humidity) and with a lifetime of at least 5000 hours with 10 000 start/stop cycles. However, as a result of their chemical and physical structure, perfluorosulfone membranes do not have the required properties to be used under these conditions. Specifically, their working temperature is limited to 80° C. Beyond this temperature, the mechanical properties of these membranes break down and, in sparingly humidified gases, their proton conductivity decreases. Moreover, even at 80° C., their lifetime does not at the present time exceed 2000 hours, on account of chemical attack that these membranes undergo by the free radicals generated by the cell, but above all on account of the mechanical fatigue generated by the successions of swelling/deswelling during the start/stop cycles. Finally, as a result of their method of synthesis and of their chemical composition, their manufacture remains too expensive for this application.
Thus, PEMFC electrolytes have been the subject of intense research worldwide for several years, in the aim of reducing their cost and/or of increasing their lifetime or their performance or their working temperature. However, it proves difficult, or even impossible, to simultaneously achieve all these objectives with the research approaches currently adopted, which are based on various improvements of the existing polymer electrolytes by physical modifications (addition of inorganic compounds, mixtures of polymers, etc.) or chemical modifications (modification of the chemical structure of the polymer to increase the glass transition temperature).
Hydrocarbon-based polymer membranes have a lifetime in a fuel cell that is more limited than perfluorosulfone membranes. The reason for this is that they are much more sensitive to chemical degradation by radical attack and have poor mechanical properties, especially on account of excessive swelling. Specifically, to have a proton conductivity similar to that of perfluorosulfone membranes, they must have a high ion-exchange capacity (IEC), which induces large swelling in the presence of water and great fragility in the dry state.
Composite or hybrid membranes based on perfluorosulfone polymers do not solve the problem of the cost of the electrolyte.
Composite or hybrid membranes based on nonconductive polymers (PBI, PVDF, etc.) doped with organic compounds (ionic liquids, etc.) or proton-conducting inorganic compounds (phosphoric acid, heteropolyacids, etc.) have elution problems and/or are limited to operating temperatures above 100° C. Moreover, it is difficult to make suitable active layers due to lack of a proton-conducting binder of the same nature as that of the membrane.
The current situation is thus that no membrane satisfies the working conditions for motor vehicle application.
The invention more particularly relates to the preparation of a membrane formed from a first network of cationic-conducting polymer (crosslinked cationic-conducting polymer referred to hereinbelow as A) and from a second network of fluorocarbon polymer (crosslinked and referred to hereinbelow as B). Since these two networks are not linked together via covalent bonds, they are referred to as interpenetrating polymer networks (IPNs).
Interpenetrating polymer networks (IPNs) have been known for a long time, and some have been described for their use in the manufacture of fuel cell membranes.
International application WO 2005/003 237 describes IPNs based on silicones and mentions the application as membrane for a fuel cell. The IPN therein is formed from a silicone-based network that may be fluorinated. The IPN is made by impregnation, using a solvent or via a supercritical route, of precursor monomers of the second network in the crosslinked or noncrosslinked silicone. The monomers are then polymerized/crosslinked to make the second network. Thus, the production of the polymer (noncrosslinked) or network (crosslinked) A takes place once the first polymer (noncrosslinked) or network (crosslinked) is formed.
However, silicones have the drawback of not being stable under the operating conditions of fuel cells. In addition, the formation of the two networks successively presents drawbacks: specifically, it is difficult to be able to impregnate the network already formed with monomers of the second network. It is possible to do so via the supercritical route, as is envisaged in said document, but this synthetic route is burdensome to implement. In addition, only a simultaneous formation of the two networks makes it possible to obtain nanometric distribution of the two phases in the case of the polymers of the invention.
Document U.S. Pat. No. 7,176,247 describes an IPN intended for DMFC use, this IPN being formed from a first proton-conducting network of sulfonated or phosphonated AMPS (2-acrylamido-2-methylpropanesulfonic acid) copolymer and from a second network of PVA (polyvinyl alcohol). These two networks are not stable under the working conditions of the fuel cell. The two networks are not synthesized simultaneously via an in situ synthesis. Finally, the synthesis described in this prior art takes place in a mixture of water and alcohol and cannot be transposed to make the IPNs that are the subject of the present invention.
Document WO 98/22989 describes fuel cell membranes composed of polystyrenesulfonic acid and of poly(vinylidene fluoride). They are prepared via a process according to which an inert PVDF matrix is first prepared and this matrix is then impregnated with the mixture of styrene and DVB. Polymerization of the styrene/DVB network allows its interpenetration with the PVDF matrix. It is followed by a sulfonation step. However, PVDF is not crosslinkable, and so the PVDF matrix does not constitute a “polymer network” within the sense of the invention. The process taught by WO 98/22989 affords access only to a semi-IPN (network interpenetrated with a polymer).
The document from T. Yamaguchi et al., Journal of Membrane Science, 214, 2003, 283-292 describes a method for preparing cation-exchange IPN membranes. Network A is created in a crosslinked porous membrane (network B). In this prior art, the porous membrane formed from a polyethylene network (CLPE) is impregnated with poly(tert-butylacrylamidesulfonic acid) (PATBS), which is then crosslinked. It is possible to replace CLPE with a crosslinked or noncrosslinked fluorinated polymer, and the PATBS with a cation-exchange ionomer containing aromatic groups. However, the method for producing the two networks described in this prior art is not simultaneous and is not performed in situ. Furthermore, the physical structure of the IPN obtained with the method described in this prior art is not at the scale of a few nanometers.
Specifically, it is known how to characterize the distance between two IPNs by measuring the minimum distance between two successive nodes. A node may be defined as the place where two networks cross (cf. FIG. 1, which illustrates two networks A and B crossing at two successive nodes labeled X separated by a distance d). This distance may be measured via various methods that are well known to those skilled in the art, such as:                small-angle neutron or X-ray scattering,        transmission electron microscopy (TEM),        atomic force microscopy (AFM).        
And, in the prior art, this distance is greater than 50 nm.
In T. Yamaguchi et al., the morphology of the material is imposed by that of the network B. Moreover, the method described in this prior art only makes it possible to obtain a material in the form of a membrane, in contrast with the method of the invention. Finally, this process, if it were applied to the polymers used in the invention, would have major implementation difficulties: specifically, it is difficult to impregnate a highly hydrophobic fluorinated porous membrane with hydrophilic precursor monomers. This requires several steps and often leaves residual porosity.